Methods to simulate textural evolution of mature grain-size distributions in the offshore zone.

The grain-size of mature coastal sands changes rapidly in response to the v ariation of the energy regime which provokes either erosion or deposition o f fractions of the textural assemblage. Whatever the dynamic conditions, qu estions of paramount importance such as the size of grain populations trans ported simultaneously, the most common size of the grains in movement and t he grain-size range of the sediment fraction involved ape still poorly unde rstood. The present paper deals with an attempt to describe this fraction w ithout any consideration of the processes responsible for the observed chan ges. A major problem results from the fact that the distribution of the mob ilized fraction cannot be known and described by direct comparison between the initial and the final grain-size curves. The only way to characterize t he mobile fraction between two subsequent grain-size states is to simulate various kinds of removal or deposition. The first question is to express th e shape of the actual distributions of the sediments mathematically The sim ple Gaussian approximation being irrelevant, distributions can be expressed using advanced computer programmes which can provide mathematical expressi ons for all distributions. The cubic-spline approximation was chosen. The m obile fraction was assumed to have a Gaussian distribution. Various tests w ere made in order to simulate qualitative phenomena observed in nature (bim odality, grain-size parameter changes...) and to assess the three parameter s which define the characteristics of the fraction deposited or removed: mo dal value, dispersion of the mobile population and relative amplitude of th e change. An application was carried out on the Senegalese coast offshore f rom the Senegal delta and on the Mediteranean coast in the Gulf of Lions. H t suggests that the dynamic agents have a great selectivity. Further develo pments are considered. (C) Elsevier, Paris / Ifremer / Cnrs / Ird.